1. Field of the Invention
The present invention relates to a process for neutralizing the tube of a radio broadcasting transmitter of high power digital type and a radio broadcasting transmitter implementing this process.
2. Discussion of the Background
The radio broadcasting transmitters currently used at short wavelengths are optimized so as to have very high efficiency when transmitting under pure amplitude modulation with carrier residual.
This optimization is achieved in particular by using a transmission tube of high power acting essentially as a switch controlled in tempo with the carrier wave to be transmitted. The transmission tube acts on a high-voltage signal which is provided by a modulator whose structure is tailored to the modulation used (PWM modulations, the abbreviation standing for xe2x80x9cpulse width modulationxe2x80x9d for example) and which is proportional to the instantaneous amplitude of the HF signal. The new radio broadcasting systems undergoing standardization are of digital type so that the form of the wave which they transmit has no immediate relationship with that of the audio frequency signal.
The use, in these radio broadcasting systems, of the same tubes as before associated with one or more modulators, whilst also wishing to transmit such a waveform demands, on the one hand, that the reference carrier be phase-modulated and, on the other hand, that the neutralization of the tube be carried out very well. In view of the geometrical constraints generally imposed on the construction of the final stage of the amplifiers, the neutralization performed according to present-day methods, which rely on analog compensation circuits, cannot be totally effective.
The aim of the invention is to propose a process tailored to a digital-type radio broadcasting system for remedying the aforesaid drawback.
For this purpose, the subject of the invention is a process for neutralizing the tube of a radio broadcasting transmitter, of high power digital type, comprising an amplifier working at saturation with a gain V1 and amplifying a phase-modulated input signal emanating from a modulator modulating at a transmission frequency two quadrature signals so as to control the grid of the tube, the latter working at saturation with a gain V2, characterized in that it consists in pre-distorting the input signal by digital means so as to compensate for the undesirable coupling between the anode and the grid of the tube of the transmitter in such a way that the output of the transmitter corresponds to a desired output signal.
The subject of the invention is also a radio broadcasting transmitter implementing the aforesaid process.
The advantage of the invention is that the compensation for the interference between the anode and the grid is carried out in a digital manner. This characteristic makes it possible to circumvent geometrical constraints which generally accompany the design of the final amplifying stage. The digital nature also allows fully tailored, flexible and upgradable compensation which requires no unwieldy modification of the transmitter.
According to a preferred mode of implementation of the process, the determination of the transfer function H(t) of the anode/grid coupling is carried out at regular instants. This mode makes it possible to obtain neutralization which takes into account the drift phenomena (due for example to aging, to heat) which are manifested in particular by phase rotations.